Many types of adhesive formulations, especially pressure sensitive adhesives based on synthetic polymers, such as acrylic polymers and styrenic block-copolymers, usually need some kind of tackifier in the formulation. The tackifier improves the adhesive properties such as tackiness, peel-resistance and ease of surface wetting. The tack of an adhesive enables it to form an immediate bond between contacted surfaces when they are brought together.
Tackifiers are generally based on resins, tackifier resins, which can be considered as high viscosity solvents for a base polymer in a formulation. A tackifier should be seen as a material which relates to a specific material or group of materials that it tackifies.
Various resinous materials have been suggested as tackifier resins for synthetic polymers. Examples of materials suggested include rosin, disproportionated rosin, rosin esters such as glycerol esters of rosin and pentaerythritol esters of rosin, aliphatic hydrocarbon resins, aromatic petroleum resins, DCPD resins, terpene resins, terpene/phenol resins and cumarone/indene resins.
Tackifier resins for pressure sensitive adhesives are usually based on rosin and/or derivatives thereof or hydrocarbon resins. Rosin is a natural resinous product mainly consisting of rosin acids. Typical rosin acids are, for example, abietic acid and levopimaric acid. The difference between the rosin acids is mainly the number and position of the double bonds. Rosin may be classified due to the species of the tree from which it originates such as Chinese, Indonesian, and Brazilian rosin and/or to the production process from which it originates such as gum rosin, wood rosin or tall oil rosin.
Hydrocarbon resins are generally low-molecular weight aliphatic or aromatic polymers typically having a weight average molecular weight of 500-3000. Common aliphatic hydrocarbon resins are resins derived from so called C5 feed streams and common aromatic hydrocarbon resins are derived from C9 feed streams. These feed streams originate from the oil cracking industry.
In aqueous adhesive compositions it is common to include the tackifier resin in the form of an aqueous dispersion. In order to make a dispersion from a resinous material such as rosin or hydrocarbon resin, a surfactant is needed acting as an emulsifier. In the case of rosin and hydrocarbon resins, the aqueous dispersion is usually made by first melting the rosin or hydrocarbon resin and then adding a surfactant and water to form a dispersion in which the rosin/resin is the dispersed phase and water the continuous phase.
Many different surfactants are known in the art such as alkyl sulphonates, fatty alcohol sulphates, and phosphate esters. However, the use of these types of surfactants may result in an adhesive, when dried, being hygroscopic which may result in an unwanted whitening of the adhesive film. Also, the cohesive strength of an adhesive bond and mechanical stability of the tackifier resin dispersion can be quite low.
U.S. Pat. No. 5,552,519 discloses a surfactant derived from a rosin ester.
US 2003/0092829 discloses a binder based on a reaction product of soybean oil, maleic anhydride and a polyethylene glycol of low molecular weight.
U.S. Pat. No. 6,464,764. discloses a wood protective agent comprising binder made from e.g. linseed oil and small amounts of maleic acid and polyethylene glycol.
WO 94/18260 discloses a water soluble and air-drying resin having an iodine value of between 50 and 150. The resin comprises substantially units of: A. a monoether of a polyethylene glycol having 5-100 ethylene oxide units per molecule, and B. the reaction product of: B1) one or more unsaturated fatty acids or one or more polyalcohol(s) esterified with unsaturated fatty acids with B2) an alpha, beta-ethylenically unsaturated carboxyl compound wherein the molar ratio of A:B is from about 0.2:1 to about 4:1.
GB 785474 discloses a process for the production of condensation products, comprising condensing a polyalkylene oxide or a derivative thereof, which oxide or derivative possesses in addition to a lipophilic group at least one free hydroxyl group and has a molecular weight of at least 1000, at an elevated temperature with a polybasic carboxylic acid which contains at least three carboxyl groups and at least one lipophilic group.
US 2004/0158003 discloses a process for preparing a surfactant comprising: reacting an α,β-unsaturated carboxylic acid or anhydride, or an ester thereof, with a resin comprising hydrocarbon monomeric residues, to provide an adduct; and reacting the adduct with either alkylen oxide or a carboxyl-reactive poly (alkylen oxide) to provide the surfactant.
Important properties for a tackifier dispersion are its foam behaviour, solids content/viscosity balance, mechanical stability, wetting properties, and shelf life.
Smaller rosin/resin particles in the tackifier dispersion will, generally, give better stability which means longer shelf life.
Thus, there is still a need for a tackifier dispersion which gives an improved water-resistance of a dried adhesive when used in adhesive compositions compared to traditional surfactants. There is also a need of providing a tackifier dispersion fulfilling the requirements listed above.
It is accordingly an object of the present invention to provide a tackifier dispersion with high storage stability, good mechanical stability and low foaming characteristics, and which only gives low loss in cohesive strength when formulated into adhesive compositions. It is a further object of the present invention to provide an emulsifier that can be produced in a process only requiring few steps.